1. A verb in command form with a direct object. "It" can define almost anything.
2. A term used to promote the use of an activity, often coupled with peer pressure.
3. Colloquial for sexual intercourse. Often used before one has the maturity to pronounce the word 'sex'.

1. Do it! Clean the dishes!
2. "I'm not smoking this joint..."
"DO IT! Smoke the joint! Once isn't gonna hurt!"
3. Ah, c'mon, me and Jessica do it all the time without condoms.